<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
*{padding:0; margin:0;}
.top{width:100%; height:120px; background:orange;}
#nav{width:100%; height:50px; background:red;}
.dingwei{position:fixed; top:0;}
</style>
<script src="jquery-1.11.0.js"></script>
<script>
/*	window.onscroll=function(){
	var oHeight=document.documentElement.scrollTop;	
	var oNav=document.getElementById('nav');
		if(oHeight>=120){
			oNav.className='dingwei';
			}else{
			oNav.className='';
				}
		}
*/
$(function(){
	$(window).scroll(function(){
		if($(window).scrollTop()>120){
			$('#nav').addClass('dingwei')
		}else{
			$('#nav').removeClass('dingwei');
			}
		})
	})
</script>
</head>

<body>
<div class="top"></div>
<div id="nav"></div>
<div style="height:1200px"></div>
</body>
</html>
